1. Field of the Invention
The present invention relates to a mixer of an arbitrarily polarized input signal. In particular, the invention relates to an optical mixer that uses two passes through a single nonlinear medium to achieve polarization independent mixing.
2. Description of Related Art
Nonlinear wavelength conversion can be used to measure very high-speed signals in an optical sampling oscilloscope. Nonlinear conversion crystals typically require specific polarizations of the signal under test, the pump, and the sum frequency generation (SFG) light waves. Whereas the pump light, being part of the instrument, can always be polarized correctly, the users input signal may have unknown polarization. Polarization controllers based on feedback have been used in the past but these are capable of polarization adjustments of at best several kilohertz bandwidth. Polarization interleaving has been employed so that the polarization changes at the bit rate, necessitating tens to hundreds of gigahertz of polarization-acquisition bandwidth. It is desirable for an optical sampling oscilloscope to simply display the reconstructed user signal power independent of the input polarization.
Use of a polarization beam splitter to split a pump laser beam polarized on a 45 degree axis with respect to the principal axis of the polarization into two channels is known. In this way 50% of the pump power passes into each of the two different channels, each channel characterized by a polarization orthogonal to the other. The input signal is also passed through the polarization beam splitter, and the two polarization components of the arbitrarily polarized input signal are split apart. These two channels are passed through a corresponding nonlinear crystal to produce a mixed signal. However, this results in a 50% reduction in conversion efficiency due to the reduced pump intensity.
The use of two nonlinear crystals employed in series so as to not suffer a theoretical 3 dB penalty is also known. However, the two crystals add to the cost, there is a need to employ an optical component between the two crystals that would correct for color and temporal dispersion, and there is a need to match the crystals, especially their temperatures.
As an example of the invention, an apparatus includes a mixer, a combiner and first and second reflectors. The mixer has first and second inputs and has first and second outputs. The combiner includes first and second parts where the first output provides a mixed signal to the first part and the second part provides a combined signal to the second input. The first reflector receives a remaining signal from the first part and provides a reflected composite signal to the second part. The second reflector receives a split off pump signal from the first part and provides a reflected pump signal to the second part.
As an alternative example of the invention, a method includes mixing into a once mixed signal, separating the once mixed signal, twist reflecting a first signal, retro-reflecting a second signal, and mixing into a twice mixed signal. The mixing into a once mixed signal mixes a pump signal of a predetermined polarity with a signal under test of an arbitrary polarity. The separating the once mixed signal separates a separated pump signal and a remaining signal. One of the separated pump signal and the remaining signal is defined as the first signal, and the other of the pump signal and the remaining signal is defined as the second signal. The twist reflecting twist reflects the first signal, and the retro-reflecting retro-reflects the second signal. The mixing into a twice mixed signal mixes the twist reflected first signal and the retro-reflected second signal.